When is it okay to check if a file exists?

Question

File systems are volatile. This means that you can\'t trust the result of one operation to still be valid for the next one, even if it\'s the next line of code. You can\'t

Answer 1

I think anytime that you know that the file may or may not exist and you want to perform some alternate action based on the existence of the file, you should do the check because in this case it's not an exceptional condition for the file to not exist. This won't absolve you from having to handle exceptions -- from someone else either removing or creating the file between the check and your open -- but it makes the intent of the program clear and doesn't rely on exception handling to perform flow-control logic.

EDIT: An example might be log rotation on start up.

  try
  {
       if (File.Exists("app.log"))
       {
           RotateLogs();
       }

       log = File.Open("app.log", FileMode.CreateNew );
  }
  catch (IOException)
  {
     ...another writer, perhaps?
  }
  catch (UnauthorizedAccessException)
  {
     ...maybe I should have used runas?
  }